Field of the Invention
The present disclosure relates to a method for producing a β-sialon fluorescent material.
Description of the Related Art
Light emitting devices including, in combination, a light source, and a wavelength conversion member, which is capable of emitting light with hues different from the hue of the light source when excited by the light from the light source, and thus being capable of emitting light of various hues owing to the principle of the mixture of colors of light, has been developed. In particular, light emitting devices composed of a light emitting diode (“LED”) combined with a fluorescent material are increasingly and widely utilized, for example, as backlights for liquid crystal displays or as lighting systems. When a light emitting device incorporates more than one fluorescent material, a fluorescent material that emits light at short wavelengths, such as blue-green, green, and yellow-green, and a fluorescent material that emits light at long wavelengths, such as orange and red, may be combined, for example, to allow liquid crystal displays to have an improved color reproduction range and lighting systems to have an improved color rendering.
As examples of such fluorescent materials, aluminate fluorescent materials, silicate fluorescent materials, sulfide fluorescent materials, phosphate fluorescent materials, and borate fluorescent materials are known. As a replacement of these fluorescent materials, fluorescent materials that have a nitrogen-containing inorganic crystal as a host crystal in the crystalline structure, such as sialon fluorescent materials, oxynitride fluorescent materials, and nitride fluorescent materials, have been proposed. Such fluorescent materials exhibit small luminance decrease associated with temperature increase and have superior durability. A representative example of such fluorescent materials is sialon, which is a solid solution of silicon nitride. α-type sialon fluorescent materials and β-type sialon fluorescent materials, which have different crystalline structure from each other, are attracting attention. In particular, β-type sialon fluorescent materials (hereinafter also referred to as “β-sialon fluorescent materials”) are highly efficient, green fluorescent materials that are excited in a wide wavelength range of from near-ultraviolet light to blue light, and have a peak light emission wavelength in the range of from 520 nm to 550 nm.
β-sialon fluorescent materials, for instance, are represented by the formula: Si6-zAlzOzN8-z:Eu (0<z≤4.2). A β-sialon fluorescent material may be obtained as a sintered product by mixing silicon nitride (Si3N4), aluminium nitride (AlN), aluminium oxide (Al2O3), and europium oxide (Eu2O3), which serves as an activator, in a predetermined molar ratio, and sintering the mixture at around 2000° C. It has been disclosed that heat-treating the sintered product in an inert gas, and acid-treating the resultant product will produce a β-sialon fluorescent material with high luminance (see, for example, JP 2005-255895 A and JP 2011-174015 A). Furthermore, to enhance the luminance of a β-sialon fluorescent material, a method of performing heat treatment of a raw material powder separately twice at a high temperature, and also a method of using a β-sialon fluorescent material obtained by sintering as a part of raw materials, are known (see, for example, JP 2007-326981 A and JP 2013-173868 A).